In the production of ethylene and lower olefins, it is known to subject a hydrocarbon, generally a petroleum fraction, to thermal degradation, referred to hereinafter as cracking, in a cracking furnace and thereafter to cool the cracking-gas mixture in a cooler or heat exchanger by indirect heat exchange with a coolant, e.g. steam or water with the recovered heat being thereby transformed into steam, usually high-pressure steam or superheated steam.
The cracking furnace generally comprises a heated chamber e.g. provided with burners, through which the cracking-gas mixture can pass via tubes (so-called cracking tubes) which can be of an undulating configuration in a large number of turns or loops at least in the cracking zone.
In the thermal cracking of hydrocarbon mixtures, especially for the production of ethylene and other lower olefins, there are significant secondary and byproduct-producing reactions which are generally technologically uninteresting because of the low value of the products produced and which must be tolerated.
These reactions can result in heavy products giving rise to carbon deposits on the walls of the cracking tubes and upon the heat-exchange surfaces of the cracking-gas cooler. These deposits of carbon, referred to as coking of the surfaces, can pose significant problems for long-term use of the apparatus. For example, they reduce the heat exchange efficiency at the externally heated coking tubes and externally cooled heat exchanger passages so that fluctuation can occur in the process parameters and a decrease in the overall energy efficiency and productivity of the apparatus. In addition, in extreme cases, the carbonization can completely block flow passages and bring operations to a standstill until these flow passages are mechanically cleaned.
Even before flow blockage, however, it is found that significant additional heat must be supplied to maintain the cracking operation, thereby increasing the energy consumption in the cracking furnace and is paralleld by a decrease in the high-pressure steam production in the cooler which is used to recover the energy invested in the cracking operation to the greatest extent possible. Eventually the process must be brought to a standstill for cleaning in the conventional approach because the loss in heat exchange efficiency in the cooler is incapable of rapidly terminating the cracking reaction which is essential to a high yield.
Consequently, it is customary to interrupt the cracking operation from time to time and mechanically decoke or decarbonize the apparatus, i.e. remove the carbonaceous deposits.
It is also possible to use a chemical cleaning approach, i.e. to introduce a mixture of air and steam to the externally heated cracking tubes and to the gas cooler. In the cracking tubes a temperature of 750.degree. to 850.degree. C. can be maintained to burn off the carbonaceous deposits. However, such temperatures are not provided within the gas cooler and hence little burnoff of the deposits in the cooler can occur. At best, inlet zones of the gas cooler show a reduction in the amount of carbon which is deposited.
The balance of the gas cooler remains contaminated with the carbonaceous deposits and hence these zones of the cooler or the cooler itself must be disconnected from the cracking reactor and subjected to mechanical cleaning.
The mechanical cleaning approach generally uses a high-pressure water jet, e.g. at a pressure of 700 to 1,000 bar, which breaks loose the deposit and scours it from the tubes.
A cleaning process of this type has been known to require about three days, during which the ethylene production is terminated, but has the additional disadvantage of generating mechanical problems for the apparatus because of the periodic reheating and cooling which is required and the expansion and contraction phenomena associated therewith which, because of the stresses they produce, reduce the useful life of the apparatus.